This kind of technique is proposed in Patent Reference-1, for example. In Patent Reference-1, as for an audio device which generates surround signals of multiple channels based on audio signals (stereo signals) of two channels corresponding to input signals, there is proposed a technique for calculating a difference between an output from an uncorrelated filter and an input signal of other channel so as to output the difference as the surround signals. Namely, the technique described in Patent Reference-1 generates a high correlation signal from the audio signals of the two channels, and makes the surround signals uncorrelated by calculating the difference.
In the specification, a signal which is reproduced from a lateral side to a rear side of a listener, or a signal which should be localized from the lateral side to the rear side of the listener is referred to as “surround signal”. Additionally, a channel of the surround signal is referred to as “surround channel”.